1. You'll want to go sight seeing. Whether taking a bus, car, or cab, or walking you'll still be doing some walking.
2. Depending on the place walking might be the best option. If you go to NYC you don't want to have to drive yourself anywhere. Subway, and cabs are great but you have to get there first - by walking. Plus in some cases walking is going to be the fastest way to get there!
3. You'll be more in shape. Everybody wants to be in shape for their vacation, right? Start walking before to help get you to your goal!
4. You'll be able to go more places in less time. If you get tired fast because you're not used walking then you won't be able to go many places, let's say again you and your family visit the big apple. You might want to go to five different places: m&m world, ripley's believe it or not museum, toys R us, Macy's and the Empire State Building. But you didn't start walking before vacation, so you go to the first two places on your list and then get too tired to go anywhere else!
5. It's a good habit to get into for after your trip! Get in the habit of walking before your trip and it'll be easier to continue when you get home.
Those are my 5 reasons, if you have anymore feel free to comment :)